Indexable insert for roughing and finishing

ABSTRACT

A cutting tool employing negative cutting inserts providing both high volume stock removal and a fine surface finish. The insert utilizes negative radial and positive axial rake angles and incorporates a lead angle of between about 30° degrees to about 90° degrees.

This is a continuation of copending application Ser. No. 07/277,601 nowabandoned, filed on Jan. 6, 1989, which is a continuation of Ser. No.07/171,332, filed on Mar. 21, 1988 still pending.

FIELD OF THE INVENTION

This invention relates to cutting tools employing lay down negativeinserts and, more particularly, to indexable inserts with negativeradial and positive axial rake angles.

BACKGROUND OF THE INVENTION

The field of cutting tools employing indexable lay down insertsencompasses a wide range of tools including face mills, step mills, endmills, boring tools and turning tools to name a few. These toolsincorporate an equally varied array of cutting edge designs toaccommodate both the operational parameters of the tools and theproduction specifications of the workpiece. Where it is desirous toremove large volumes of material (rough cutting) by face milling, thecutting edge has been designed to withstand higher loading. For example,a face milling tool employed to rough cut carbon steel, driven by a30-50 horsepower machine may operate at a feed rate of 0.015 inches pertooth at a 0.125-0.250 inch depth of cut utilizing a surface feed rateof 200-300 feet per minute. Smooth finish milling, however, placesdifferent conditions on cutting edge design. A face mill equipped withfinishing inserts and operated in the same machine and in the samematerial above runs at a lower feed rate of 0.005 inch - 0.008 inchesper tooth and at 0.020-0.030 inch depth of cut at 300-400 feet perminute. Although the cutting edge in finish milling is not required towithstand the same load requirements as in rough cutting, the edge mustprovide a considerably smoother surface finish. Values in the rangeabout 125 RMS for fine surface milling in comparison with about 250 RMSfor rough cutting are not uncommon.

Several attempts have been made to improve cutting edge performance inboth rough and finish cutting tools by changing the orientation of thecutting edge with respect to the tool seat. Though varied, theapproaches have included the selection of a positive or a negative rakedesign.

Negative rake cutting tools provide an insert seat which is inclined ata negative rake angle relative to the cutting plane and a straightsidewall form. The inclination of the insert seat assures clearanceunder the cutting edge.

Positive rake cutting tools provide inserts fixed to inclined seats insuch a manner that inserts are required to have an inclined sidewallform to provide clearance under the cutting edge. The back wall of theinsert must be inclined rearwardly to complement the sidewall form ofthe insert. This provides a ramp surface rather than a pocket at theback wall of the insert and requires additional means for fastening theinsert. Most positive rake inserts provide only half the number ofavailable cutting edges because they cannot be indexed end over end. Useof a negative rake insert is therefore desirable. Examples of artemploying positive rake inserts include U.S. Pat. Nos. 3,938,231 and3,868,752.

It is also known in the art to modify the orientation of the cuttingedge with respect to the cutting tool and workpiece by employingpositive or negative radial and axial rake angles.

Generally, the term "rake" is the angular relationship measured betweena reference plane and a reference face of the insert. The referenceplane passes through the cutter body central line axis and the insertscutting corner. The reference face sometimes referred to as rake face orfirst surface herein is the face that sees the work piece and isdependent upon the direction of cutter rotation.

The inserts radial rake angle is the angle formed by the referencedplane and the rake face as measured in the plane perpendicular to thecutter body axis.

Radial rake is defined as positive where the rake face forms an acuteangle with respect to the reference plane such that the rake face slopesaway from the direction of cutter rotation when applied to a workpiece.Radial rake is defined as negative where the rake face forms an obtuseangle with the reference planes and slopes toward the direction ofcutter rotation. Generally, a negative rake is preferred in applicationswhere the cutting edge is be subject to high loading.

The insert's axial rake connotes the angle formed between the referenceplane and the rake face measured in a plane perpendicular to the radiusof the cutting body, at the working cutting corner. The use of negativeradial rakes in combination with negative or positive axial rakes isknown in the art. One example is found in U.S. Pat. Nos. 3,289,271.

It is also known in the art to further define the orientation of acutting edge in terms of its true rake angle and angle of inclination ortrue shear. The true rake angle is defined by drawing an imaginary linenormal to the cutting edge and intersecting the axis of the cutter body.The angle between the rake face and this imaginary line is the true rakeangle.

The angle of inclination is defined by drawing an imaginary line throughthe center point of the cutter body and tangent to the radiallyoutermost point of the cutting edge. The angle between this line and thecutting edge is the angle of inclination.

If the plane of the rake face passes through the cutter axis, the truerake is said to be zero. If the top working corner of the cutting edgeis ahead of the lowermost point on the cutting edge, the true rake issaid to be positive. If the radially outermost point of the cutting edgepasses through the cut first, then the inclination angle is said to benegative.

GTE Valenite U.S. Pat. No. 4,352,609 discloses a face milling cutter anda cutting edge with a positive true rake angle in the range of 0° to 3°and a radial rake on the order of 0° to 2° positive, with an axial rakeon the order of 4° to 6° negative. A cutting edge utilizing a positiveradial rake with a negative axial rake, however, tends to not onlyfracture under heavy loading but also directs spent chips toward theworkpiece. This results in recutting and scouring, requiring greaterhorse power per cubic inch of stock removed.

Attempts at modifying the performance characteristics of cutting toolshas also included incorporating a lead angle. The term lead angle isknown in the art and is defined as an angle formed between the radiallyoutward facing insert edge that includes the working cutting corner andan imaginary line oriented parallel to the cutting body axis that passesthrough the cutting corner.

A cutting tool, which has high efficiency in both soft and difficult tomachine materials and facilitates the formation of tight chip for rapidremoval and also generates a smooth surface at high feed rates would bea desirable advance in the art of cutting tools. By optimizing theradial and axial rakes of the insert, the extent of the lead angle, truerake and angle of inclination, the problem of stocking multiple cuttersand inserts for rough cutting and finishing is alleviated.

One object, therefore, in the present invention is to provide a cuttingtool employing lay down indexable inserts which includes an enhancecutting edge and which generates short, tightly curled chips.

A further object of the present invention is to provide a cutting toolemploying a lay down indexable insert which can withstand high feedrates in difficult to machine materials without failure andsimultaneously deliver a smooth final finish on the order of 125 RMS orbetter.

A further object of the present invention is to provide a cutting toolemploying a lay down indexable insert which is easy to manufacture andwhere the dies for the insert are made according to standard machiningpractices.

SUMMARY OF THE INVENTION

Accordingly, these objects are accomplished by a cutting tool having abody with a central axis where the body has at least one recessed pocketdisposed about the axis for receipt of at least one cutting insert. Theinsert comprises a wafer of cemented carbide having parallel polygonalshaped top and bottom faces. The insert is mounted in the recessedpockets by securing means. Both the top and bottom face is separatedfrom the other by at least three peripheral side sections whichintersect to form at least one acute angle with an adjoining side. Eachof the side sections includes an upper first and lower second planarsurface which intersect at an included obtuse angle to form a line whichis parallel to imaginary planes passing through the top and bottomfaces. The first surface of one of the peripheral adjoining sidesintersects with the top face at a right angle. The first and secondsurface of at least one of the adjoining peripheral side sectionsintersect the top face at an obtuse angle and the bottom face at a rightangle, respectively. The intersection of adjoining first surfaces at anacute angle creates at least one cutting edge having a negative radialrake and a positive axial rake. The cutting edge also employs a positivelead angle, a negative true rake and a positive angle of inclination.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the invention will becomeapparent from a reading of a detailed description of a preferredembodiment taken in conjunction with the drawings in which:

FIG. 1 is a fragmentary axial view of a milling cutter fashioned inaccordance with the principles of the invention;

FIG. 2 is an elevated schematic side of a milling cutter illustratingoverlapping fields of cutting;

FIG. 3 is a schematic side elevation of a milling cutter illustratingthe location of a single cutter insert in relation to a fragmentaryworkpiece;

FIG. 4 is a schematic mounting face view of an insert having paralleltop and bottom faces, parallelogram in form;

FIG. 5 is a view of a side face taken from the direction of line 1--1 ofFIG. 4;

FIG. 6 is a perspective view taken from the direction of line 2--2 of anoutermost side cutting corner of FIG. 4;

FIG. 7 is an edgewise view normal to at least one cutting edge of thecutter of FIG. 1 illustrating a lead angle of the present invention;

FIG. 8 is a view taken from the direction of line 3--3, of FIG. 7,illustrating a radial rake angle;

FIG. 9 is a view taken from the direction of line 4--4, of FIG. 7,illustrating a true rake angle of the present invention;

FIG. 10 is a view taken from the direction of line 5--5, of FIG. 7,illustrating an axial rake angle, and

FIG. 11 is a view taken from the direction of line 6--6, of FIG. 7,illustrating an angle of inclination of the present invention.

FIG. 12 is a view showing the cutting insert with scallops.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1 and 3, a cutting tool 1 is illustrated having a body10 designed for rotation about a longitudinal axis 20. The body includesat least one recessed pocket 25 which is designed for receipt of anindexable insert 50. FIG. 1 illustrates a face milling tool where theinserts are spaced at equivalent radial distances from the center axis.It should be readily apparent to those of ordinary skill in the art thatthe use of a face mill is for illustrative purposes only and the presentinvention could just as easily be adapted to any number of cutting toolsmentioned above including those which stagger the orientation of theinsert pockets to provide overlapping fields of cutting such as shown inFIG. 2. It should also be apparent to those of ordinary skill in the artthat the present invention could just as easily be employed in turningtool applications where the cutting tool remains stationary and theworkpiece is rotated or in broaching applications.

Referring to FIG. 3, each pocket is milled from the tool body bystandard milling techniques to include a seat face 30 and at least oneabutment wall 35. Preferably, a second abutment wall 40 is milled fromthe tool body to provide adjoining abutment wall surfaces. Seat face 30is generally planar and includes an aperture which is threaded for areceipt of a retaining screw (not shown). It should be apparent that thepresent invention is not limited to any particular means of fixinginsert 50 in pocket 25. Reference hereafter will be made to screw whichpasses through aperture 55 and threads into the beforementioned aperturein seat face 30 to force the insert 50 into a secure abuttingrelationship with walls 35 and 40. The insert could just as easilyemploy any number of pin or clamp designs commonly available in the art.The present invention also recognizes that various brazes commonly knownin the art may be employed to secure the insert. One particularly brazeis disclosed in U.S. Pat. No. 4,389,074 assigned to GTE ProductsCorporation.

Insert 50 is a wafer of hardened material selected from the groupincluding cemented carbides such as tungsten carbide, silicon nitride orCeramic or other materials sufficiently resilient to withstand theforces encountered in stock removal such as cast iron carbon steel,alloys of nickel, chrome materials, aluminum, and exotics such astitanium or composite materials. Preferably VC55 tungsten carbideproduced by GTE Valenite is used in steel applications. Other carbidegrades are available from GTE Valenite the selection of which depends inpart on the application to which the tool is applied.

Although the configuration of the insert is generally polygonal in formand therefore may include triangular, circular, or square geometries,reference hereafter will be to a parallelogram for purposes ofconvenience. As shown in FIGS. 3, 4, and 5, insert 50 includes anaperture 55 and recessed conical portions 60, 65 located in top face 70and bottom face 75. The top and bottom faces are generally parallel.

Each face 70, 75 is parallelogram in form with at least one andpreferably two pairs of oppositely oriented corner sections 85, 90. Eachface includes four-edge surfaces 105, 110, 115, 120 which intersect atoppositely oriented pairs of corners 85, 90 to form oppositely orientedacute and obtuse included angles of between about 0° and 90° and 90° and180°, respectively.

Referring to FIGS. 4, 5 and 6, the top and bottom faces 70, 75 areseparated by at least three and preferably four adjoining peripheralside sections 125, 130, 135, 140. In one embodiment, each side adjoinstwo other sides at corners 85, 90 to form two pair of sides opposed inorientation where each side 125 forms an acute included angle 145, andan obtuse included angle 155 with adjoining sides 120, 130. It isgenerally recognized to those of ordinary skill in the art that therange of the acute and obtuse angles may vary and are dependent on theshape of the insert and the application.

Each of the side sections include a first upper surface illustrated inFIG. 6 at 165, 170, 175, 180, and a second lower surface at 185, 190,195, 200. At least one pair of oppositely oriented first surfaces 170,180 are parallel to each other and intersect with the top face 70 tosweep out right angles. At least two second surfaces 185, 195 areoppositely oriented, parallel to each other, and intersect with bottomface 75 to sweep out right angles. First surfaces 165, 175 (and 170,180) form included obtuse angles with respect to top face 70 (and bottomface 75) of between about 91° to about 179° degrees. A range of betweenabout 91° to about 140° degrees in conjunction with a lead angle of0°-45° degrees is preferred when optimum surface finish is desired. Eachrespective side section surrounds the periphery of the insert 50 withalternating planar first section forming right and obtuse angles withrespect to top and bottom faces 70, 75. For each individual side, therespective first and second surfaces intersect to form a line 203 whichis parallel to imaginary planes passing through the top face 70 andbottom face 75. On one index, the first surface 170 and the secondsurface 195 may be secured against adjoining abutment walls 35, 40 (seeFIGS. 1, 3), when seated securely in pocket 25.

First surface 175 of side 135 intersects with first surface 170 of thesecond side 130 at corner 85 and forms one of at least two oppositelyoriented cutting edges. The number of cutting edges selected, however,is dependent in part on the shape of the insert.

In an alternate embodiment, the oppositely oriented pair of firstsurfaces adjoining the vertical planar first surfaces referenced aboveare crowned to facilitate fine finished turning. The crown is formed bya radius calculated in accordance with the formula ##EQU1## where Cconstitutes the width or chord of the first section extending betweenthe top face and its intersection with the second face, and Hconstitutes the height of the crown. An insert with a 5/8 I.C. and 5/16inch thick, employs a crown height of between about 0.0002 inches toabout 0.0015 inches. The width of the crown C is dependent in part uponthe size of the insert selected. A radius of about 2.813 inches isemployed where a chord width of 0.150 and a height of 0.001 areselected. The radius is measured perpendicular to the chord. Themagnitude of the radius is dependent in part on the degree of surfacefinish desired. The embodiment alleviates the problem of a wavey orshingled surface finish caused by spindle tilt.

In another embodiment, the insert may be scalloped to facilitate smallerchip width and to reduce horsepower requirements. A scalloped edgeportion is oriented on the clearance face of the insert defined alongthe edge created by the intersection of the top face with a firstsurface or rake face which extends generally perpendicular thereto. Thescalloped sections may be oppositely oriented where the insertconfiguration includes parallelogram shape top and bottom faces. Thescallops may be produced on the insert by manufacturing methods known inthe art such as disclosed in U.S. Pat. No. 4,606,678, column 1, lines65-68 and column 2, lines 1-25 of which are hereby incorporated byreference. Other methods are known in the art and will not be furtherdiscussed herein. The number of scallops per section is dependent on thesurface finish desired. The greater the number of scallops, the finerthe finish and the lower the horsepower requirements for machining. Eachinsert side, when mounted in a tool body, is phased differently from theremaining inserts to facilitate the removal of the high and low pointsformed by the scalloping. By staggering the orientation of the scallopedsection, overlapping fields of cutting generate a smooth flat surface.

In another embodiment, the insert may include a crowned surface asdisclosed above on the first surface (or rake face) in addition to thebeforementioned scalloping.

In yet another embodiment, the intersection of first and second surfacesper side form a line which is skewed toward top face 70 or bottom face75 depending on the index. This serves to strengthen the cutting edgedue to the additional material of the enlarged second surface. Anenlarged length of cut is also provided due to the diminished portion ofthe first surface adjoining the cutting edge. Alternatively, the linemay be skewed to provide an increasing section of the first surface.This serves to increase both the axial rake and the angle of inclinationfacilitating cutting in soft materials such as low silicon aluminum,soft brass or lead.

The orientation of the cutting edges with respect to the insert's radialand axial rake angles, lead angle, angle of inclination and true rakeangle, is particularly important to the present invention. With regardto FIG. 8, insert 50 has a negative radial rake angle 235 formed by theintersection of first surface 170 and top face 70 as measured in animaginary plane 250 oriented perpendicular to the cutter body axis 20.Top face 70, as illustrated, slopes toward the direction of cutterrotation 255 when applied to workpiece (not shown). A negative radialrake of 0° to 30° augments cutting edge strength due to the 90° includedcorner. As stated above, the 90° edge is formed by the intersection ofthe first surface 170 and the top insert faces 70. Clearance is providedby the negative radial rake angle. As the cutting edge approaches 0°radial, the clearance angle lessens. In particular, a radial rake anglebetween 10°-20° is preferred. A rake angle rake beyond 30° causes thetrue rake to become too negative to cut efficiently as the cutting facebegins to become increasingly tangent to the work surface.

Insert 50 is oriented with a positive axial rake as illustrated in FIG.10. The axial rake angle 275 is formed between the intersection ofimaginary planes passing between the first surface 170 of insert 50 anda line 285 oriented parallel to the cutting tool axis of rotation 20.Preferably, the first surface 170 slopes away from the direction ofcutter rotation as illustrated by arrow 255 when applied to a workpiece(not shown). A positive axial rake angle between about 10° to about 20°,directs material away from work surface and favors the formation oftightly spiraled chips. This helps to achieve a finer surface finish byeliminating, recutting and scouring. In addition, the positive anglerequiring less horse power because the positive axial rake functions asa chisel rather than a plow.

A preferred axial rake angle range of 10° to 15° positive combined witha negative radial rake results in obtaining a positive inclination of15° to 25° degrees which provides for improved shearing of the material.As the axial rake increases beyond 20° degrees, the cutting edge becametoo frail to support heavy cutting in difficult to machine materials. Achange from 20° to 30° degrees would increase the negative radial rakeand increase the negative true rake. A more positive angle ofinclination would also be generated. If radial rake were not increased,the clearance of the flank surfaces would be thereby greatly reducedleading to heeling of the cutter and ultimately a diminished tool life.

Referring to FIG. 7, a lead angle 240 of 0°-90° degrees allows thecutter to advance a greater distance per tooth than the actual chipthickness. For example, with a 90° lead and a feed rate of 0.010 inchesper tooth, an actual chip thickness of 0.010 inch per tooth develops.With a 45° lead and a feed rate of 0.010 per tooth, a 0.007 actual chipthickness is found. The lead angle, therefore, allows for parts to bemilled faster making the tool more economical.

Enhanced results may be obtained where the cutting edge employs anegative true rake angle between about 5° to about 10°. Referring toFIG. 9, the true rake angle 290 is swept out by imaginary plane 295formed through top face 70 and first surface 170. Plane 295 is normal toaxis 20. A preferred range of 7° to 8° has been determined throughtesting. As the angle approaches 10° additional horse power is requiredto maintain cutting speed over the workpiece.

Referring to FIG. 11, inclination angle 297 is defined by drawing animaginary line 300 parallel to axis 20 and passing through corner 85. Apreferred angle of 15° to about 25° directs chips away from the finishsurface of the workpiece. As the angle of inclination increases, thecutting edge becomes too fragile for rough cutting. In addition, anangle in excess of 25° degrees renders the cutting edge too brittle forexotic materials.

An embodiment of the present invention as illustrated in FIG. 1 wasconstructed for purposes of evaluation. The face mill utilized a 6 inchcutting diameter and was equipped with 12 inserts arranged about theperiphery. Inserts were formed from GTE Valenite VC55 tungsten carbidewere oriented with 21° negative radial and 10° positive axial rakeangles and with an 8° negative true rake and 21° positive angle ofinclination. Testing was done on a 50 horsepower Cincinnati verticalmilling machine using 1045 (AISI) steel with a hardness coefficient of210 bhn and a tensile strength at 120 ksi. Parameters of feed per toothtraveled, net horse power, horse power per cubic inch and surface finishwere measured.

                  TABLE I                                                         ______________________________________                                        Cut No.  FPT    Net HP       HP/IN3 RMS                                       ______________________________________                                        1        .0056  15.2         .95    63                                        2        .0064  16.9         .92    50                                        3        .0079  20.2         .86    52                                        4        .0092  22.2         .84    42                                        5        .0114  27.3         .83    27                                        6        .0131  30.5         .81    35                                        7        .0158  36.9         .81    58                                        8        .0183  40.9         .78    68                                        9        .0228  52.1         .79    86                                        ______________________________________                                    

As illustrated above, the embodiment produced mean value results of 84.3for horse power per cubic inch milled and 53.4 RMS for surface finish.

It will be understood that other embodiments and modifications of theinvention are contemplated. It is the intention to include all suchembodiments and modifications within the scope of the invention as aredefined by the appended claims.

I claim:
 1. A cutting tool comprisinga tool body rotatable about acentral axis, said body having recessed pockets disposed therein forreceipt of indexable cutting inserts, said pockets disposed about saidaxis in a staggered array which provides for overlapping fields ofcutting, means for securing said inserts in said pockets, said insertcomprising a wafer of hardened material with polygonal shaped top andbottom faces, said faces oriented parallel to each other and separatedby at least four adjoining peripheral side sections, said four sidescomprising two pair of opposed sides where each side forms an acute andan obtuse angle with adjoining sides, each of said sides furthercomprising first and second planar surfaces which extend the entirelength of the side and intersect with each other to form an obtuseincluded angle, said intersecting planar surfaces forming a lineparallel to imaginary planes passing through said top and bottom faces,said first planar surface of one side intersecting said top face at aright angle, a plurality of scallops intersecting each other andpositioned along said right angle intersection opening from said topface toward said first planar surfaces, said first and second planarsurfaces of at least one adjoining side intersecting said top face at anobtuse included angle and said bottom face at a right anglerespectively, said adjacent first planar surfaces intersecting at anacute included angle to form at least one cutting edge, said inserthaving at least two oppositely oriented cutting edges per index, saidcutting edge having a negative radial rake and positive axial rake, saidcutting edge having a positive lead angle, and said cutting edge havinga negative true rake and a positive angle of inclination.
 2. The cuttingtool of claim 1 wherein said pockets comprise a planar seating face andinclude at least two adjoining abutment walls.
 3. The cutting tool ofclaim 2 wherein said seating face includes a threaded aperture.
 4. Thecutting tool of claim 3 wherein said bottom face is secured in saidpocket seat.
 5. The cutting tool of claim 4 wherein said inserts aremade of tungsten carbide.
 6. The cutting tool of claim 5 wherein saidinserts include an aperture which intersects said top and bottom facesand further includes conical sections recessed in said top and bottomfaces.
 7. The cutting tool of claim 6 wherein said mounting meanscomprises a screw which threads into said seating face and forceablysecures said top or bottom face against said seating face.
 8. Thecutting insert of claim 7 wherein said top and bottom faces areparallelogram in shape with at least two pair of parallel sides.
 9. Thecutting insert of claim 8 wherein said cutting edge has a lead angle ofbetween about 0° degrees to about 90° degrees.
 10. The cutting edge ofclaim 9 wherein said negative radial rake is between about 0° degrees toabout 30° degrees.
 11. The cutting tool of claim 10 wherein saidpositive axial rake is between about 0° degrees to about 20° degrees.12. The cutting tool of claim 11 when said negative true rake is betweenabut 5° degrees to about 10° degrees.
 13. The cutting edge of claim 12wherein said positive angle of inclination is between about 0° degreesto about 30° degrees.
 14. The cutting tool of claim 13 wherein saidscallops are staggered across the top and bottom face to provideoverlapping fields of cutting.
 15. The cutting tool of claim 14 whereinsaid scallops are angular to the cutting edges and when phased in thecutter body, it provides staggering of the scallops to produceoverlapping field of cutting.